git-rev-list.txt: Clarify the use of multiple revision arguments

git-rev-list.txt: Clarify the use of multiple revision arguments

If one thinks of a revision as the set of commits which can be reached
from the rev, and of ^rev as the complement, then multiple arguments to
git rev-list can be neither understood as the intersection nor the union
of the individual sets.

But set language is the natural as well as logical language in which to
phrase this. So, add a paragraph which explains multiple arguments using
set language.

Suggested-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>